Precious metals are of significant technological importance. They include elements such as platinum, gold, ruthenium, rhodium, palladium, rhenium, iridium, osmium and the like. The technological importance of precious metals is illustrated by their many and diverse applications in the areas of catalysis, electronics, and jewelry. The oxidation resistance of these precious metals makes them useful for high temperature applications. In metallic form, precious metals are biocompatible as well as highly corrosion resistant. This makes them useful in medical devices and dental applications. Precious metals are extremely rare, due to their low natural abundance, and the complex processes required for their extraction and refining.
Fossil fuels, such as coal, are typically used in furnaces and boilers to produce steam and ultimately electricity for industrial purposes. The steam is typically used to drive the generators and produce electricity. Although steam is the intermediate product that is required to ultimately produce power, combustion by-products result from the burning of coal in the boilers.
One type of coal burning boiler is the cyclone furnace. The cyclone furnace is a slow-heating, water-cooled furnace having a horizontal cylindrical shape in which a fossil fuel, such as coal, is fired cyclonically. As the coal is burned in the boiler, heat is produced to boil water circulating through tubes that are positioned about the outer circumference of the boiler. This process produces steam which is required to ultimately produce electricity.
The coal combustion process results in the production of by-products, such as slag. Slag comprises the molten impurities of the coal combustion process, including oxides, silicates and iron metal. The slag is collected in a collection tank under the coal burning furnace. The slag from the coal burning furnace is a potential source of precious metals, such as platinum, gold, ruthenium, rhodium, palladium, rhenium, iridium and osmium. The large quantity of cyclone slag produced is available for precious metal extraction and recovery at a low cost.
There are recovery methods known in the prior art that pertain to recovering precious metals from complex or carbonaceous ores, metal smelting slag, and used catalysts. They include electrochemical methods, pyrometallurgical methods which use reducing agents and heat-treatment, complexing agents such as cyanides, and leaching methods of recovery utilizing various acid solutions. In U.S. Pat. No. 4,268,363, to Coughlin et al. discloses electrochemical gasification of carbonaceous materials to produce oxides of carbon at the anode, and hydrogen or metallic elements, including precious metals, at the cathode of an electrolysis cell.
U.S. Pat. No. 4,997,532, to Flax et al., describes a method in which noble metals are extracted from raw materials by first leaching the materials with mineral acid (leaving the noble metals in the residue), then treating the residue with a non-aqueous liquid containing bromine to form a complex of the formula R[MeBrx], and subjecting this complex to electrolysis whereby the noble metal is deposited on the cathode.
Precious metals are recovered from iron-bearing slags created in nickel and copper smelting processes by a method disclosed in U.S. Pat. No. 4,717,419. The slags are treated under reducing conditions at temperatures of 1100-1400.degree. C., then further reduced by sulphidization. The molten slag is then cooled under controlled conditions. After crushing and grinding, the precious metals, which have been concentrated into the iron-base metal phase, can be separated by means of magnetic separation.
U.S. Pat. No. 5,238,485 to Shubert et al., discloses that precious metals can be recovered from complex ores by heating the ore to the molten state with a reducing agent, fluxing agent, and an iron-embrittling agent. After cooling, the precious metals are separated through use of an electrolytic process or selective chemical dissolution.
Kohr discloses a method in U.S. Pat. No. 5,626,647 in which precious metals are recovered from carbonaceous ore by means of a leach solution containing cyanide, aqua regia or thiourea.
U.S. Pat. No. 4,404,022, to Godbehere, is directed toward a method for treating dore slag. Dore slag is slag from a dore furnace, which is used to smelt slime material from the electrolytic refining of copper, to produce dore metal, consisting essentially of silver and gold. The method comprises the steps of grinding the slag, mixing the obtained pulp with water to form a slurry, adding a suitable collector, such as sodium di-isobutyl dithiophosphate, adding a suitable frother, such as methyl iso-butyl carbinol, aerating the slurry, and separating the precious metals-containing froth from the remainder of the slurry
There are problems with each of the methods cited in the known prior art. There are safety issues raised when using strong mineral acid compounds. Specialized equipment is required for electrochemical treatment or electrolysis. Large furnaces are needed to heat slag to the molten state. Frother and collection compounds can be both toxic and costly. It is still desirable to develop a method to recover precious metal from slag produced by a coal burning furnace which does not require electrolysis or the addition of concentrated, toxic, and costly chemical reagents, which does not require heating the slag, and which will be economically and environmentally more desirable.
The United States, which currently imports a large percentage of its precious metal requirements, will be increasingly less dependent on foreign supplies of precious metals. The recovery of precious metals from coal furnace slag will help to reduce the need for mining and, thus, reduce associated environmental problems. In addition, the current cost of disposing of coal burning cyclone furnace slag will be somewhat mitigated by the realization of the value of the precious metal content of the slag.